One of the properties required for the polyester film is transparency. However, simultaneously, slipperiness, which is requisite for easiness in handling of the polyester film, is required. As means for providing the film with such characteristic, it is known to provide a slippery layer on the surface thereof by coating or to incorporate a lubricant in the polyester. These measures are not satisfactory from the viewpoint of reliability and manufacturing cost. Therefore, up to now, the problem has been tried to be solved by incorporating fine particles in monolayer films and conditions of the suitable particles have been searched for. Use of amorphous silica is disclosed in Laying-Open Publication Sho 61-24978 for instance. An attempt to reduce the degree of planar orientation in film making is described in Laying-Open Patent Publication No. Sho 61-293832.
Even by these measures, however, it is difficult to satisfy the today's strict demand. When such silica particles are used, transparency of film is apt to be impaired by voids formed around the incorporated particles and the film cannot be free from a defect called "frosting" which is caused by a wide particle size distribution of used silica. The reduction of degree of planar orientation is inherently insufficient in improvement of slipperiness and it requires a film composition which reduces the speed of film making, which invites rise in manufacturing cost.
In view of the above-described situation, we conducted an extensive study in search of a polyester film having excellent transparency and slipperiness, we have found that a biaxially stretched film which comprises a matrix of transparent polyester and particles of a specific cross-linked high polymer incorporated in the former are provided with excellent transparency and slipperiness.